1. Field of the Invention
The present invention relates to the improvement of an LC noise filter provided on a dielectric.
2. Description of the Related Art
A noise filter produced by providing a spiral inductor conductor 2 on the surface 10a of a dielectric substrate 10 made of a ceramic material or the like, as shown in FIG. 30(A), and forming a grounding conductor 4 on the back surface 10b of the dielectric substrate 10, as shown in FIG. 30(B) is conventionally known.
Such a noise filter provides an inductance (L) by the spiral inductor conductor 2 and an electrostatic capacitance (C) between the inductor conductor 2 and the grounding conductor 4 in the form of a distribution constant, as shown in FIG. 31 and functions as an LC noise filter.
The conventional LC noise filter, however, has the following problems.
(a) First problem
The conventional LC noise filter generates an eddy current on the grounding conductor 4 provided on the back side of the substrate 10. For this reason, the inductance provided by the spiral inductor conductor 2 is not as large as expected, and the electric characteristic obtained are not those produced by an LC noise filter but rather resemble those produced by a capacitor.
More specifically, in this LC noise filter, the grounding conductor 4 not only capacitively couples with the spiral inductor conductor 2 by electrostatic capacitance, but also inductively couples therewith. Therefore, the magnetic flux produced by the energizing current of the spiral inductor conductor 2 also produces an electromotive force on the grounding conductor 4, and a short-circuit current flows as indicated by the solid line A in FIG. 30(B) due to the electromotive force.
If the spiral inductor conductor 2 is compared to a primary coil in a transformer, the grounding conductor 4 acts like a secondary coil, so that it is impossible to obtain as large an inductance as expected from the spiral inductor conductor 2. In other words, such a conventional noise filter cannot display full function as an LC noise filter.
(b) Second problem
The noise filter removes the noise contained in a signal supplied to electrodes 6 and 8 on both ends of the inductor conductor 2. However, if the frequency of the signal supplied is high, a short circuit between lines such as those indicated by the arrow B is caused on the spirally wound inductor conductor 2, as shown in FIG. 30(A), thereby lowering the function of the inductor conductor 2 as an inductor.
The higher the frequency of the signal supplied, the more frequently such a short circuit between lines is caused. The conventional noise filter is therefore not suitable, especially as a high-frequency noise filter.
(c) Third problem
This noise filter does not produce such a large capacitance C as expected between the inductor conductor 2 provided on the surface of the substrate 10 and the grounding conductor 4 provided on the back side of the substrate 10, and the electric characteristics obtained are inferior to those of a lumped constant LC noise filter.
When the frequency of the signal supplied to the inductor conductor 2 becomes high, the inductance of the inductor conductor 2 changes so that the capacitance C between the inductor conductor 2 and the grounding conductor 4 becomes even smaller. Thus, this noise filter cannot fully function as an LC noise filter.